You can redirect domains to your main domain using the Redirect Domain Tool.
To redirect another domain to your website:
Go to your website Dashboard.
Click the Domain button and select Set unique domain from the list.
Click the Add Existing Domain button and enter the domain you want to redirect to your main domain (including www).
You will get a message that the domain was added successfully to your domain list, then click Save.
Click the Domain button again, and select Redirect Domains from the list.
Click the Redirect a new domain button.
Enter the domain (including www) and Save.
Make sure that the domain that redirects to your main domain on SITE123 is set with the following records:
A Record that points to 18.104.22.168
CNAME for www that points to ssl.site123.com